2. Exploring the Ecology of Thermophiles from Australia’s Great Artesian Basin During the Genomic Era

The Great Artesian Basin (GAB) is the world’s largest subsurface aquifer, underlying approximately one-fifth of subarid regions of the Australian continent and covering an area of over 1.7 × 106 km², with a water-storage capacity of 8.7 × 1012 m³. The GAB provides a vital water resource for rural semiarid communities and also contains the largest onshore oil and gas reserves in Australia. The GAB is composed of alternating layers of water-bearing permeable sandstone and non-water-bearing impermeable shale. These geological formations have an immense influence on the chemical composition of GAB groundwaters, which can be bicarbonate-, chloride, sulphate or iron rich. The depth of the aquifer is estimated to be 3,000 m, and the underground water flow from the recharge areas at the edge of the basin to the discharge areas in central Australia as mound springs is estimated to be 1–5 million year⁻¹. The water is heated by the Earth’s magma due to its depth, and the age of the water is calculated to be over 2 Ma. Not only do more than 5,000 free-flowing bores, with source temperatures ranging between 100 and 30°C, depending on bore depth, provide an important water resource to the outback communities, but the GAB is also a favourable environment for the growth of a wide diversity of microbial life. Distinct thriving macroscopic microbial mat communities can be seen colonising specific temperatures along the temperature gradient of runoff channels formed by the free-flowing bores. In the last two decades, a range of thermophilic and mesophilic microorganisms have been characterised from the GAB waters which include sulphate reducers, carbohydrate fermenters, strict aerobes and dissimilatory metal-reducing microorganisms (DIRM). During recent years, there has been a significant drop in the GAB groundwater pressure and volume, largely due to water leakage from corroding bores, and this is a matter of great concern. The isolation of metal-reducing microorganisms from the GAB environment suggests that they could be colonising the metal casing of such bores, thereby contributing to bore corrosion and subsequent complete bore failure. It is widely accepted that metal-reducing microorganisms have a large impact on the geochemistry of subsurface environments through the cycling of metals and organic matter and thereby affect water quality and taste. Furthermore, metal-reducing microorganisms have potential applications in bioremediation, mineral leaching and energy generation processes and are of evolutionary interest as metal reduction is considered to be a very ancient form of respiration. In this report, we provide an insight into the microbial diversity of this unique subsurface aquifer.